1. Field of the Invention
The present invention relates to a print head for use in a wire dot printer. More particularly, the present invention pertains to a stopper structure for preventing an armature from coming off an armature guide during an assembling process.
2. Description of the Related Art
FIG. 5 is a fragmentary sectional side view showing the internal structure of a conventional print head used in a dot printer.
As illustrated, the print head comprises a casing consisting of a front case 1 and a rear case 2 and a magnetic drive unit accommodated inside the casing. The magnetic drive unit comprises a yoke 3 having a substantially U-shaped cross-section, a core 4 upstanding from the yoke 3, a coil 5 wound on the core 4, an armature 6 made of a magnetic material and disposed between the respective end faces of the yoke 3 and the core 4 on the one hand and the rear case 2 on the other, a wire 7 connected to one end of the armature 6, a return spring 8 interposed between the first end of the armature 6 and the upper end of the yoke 3, a support spring 9 interposed between the other end of the armature 6 and the rear case 2, a damper 10 rigidly secured to the lower surface of the rear case 2, and an armature guide 11 mounted on and secured to the yoke 3.
The armature guide 11 has a plurality of retaining portions 11a in correspondence with the armatures 6, respectively, as partially shown in FIG. 6, the retaining portions 11a being connected together in one unit through an annular peripheral wall 11b. Since the armature 6 is formed so as to have a width slightly smaller than the spacing between a pair of adjacent retaining portions 11a, the armature 6 faces the corresponding core 4 in a state wherein the circumferential movement of the armature 6 is limited. The gap (magnetic gap) between the armature 6 and the upper end of the core 4 is set at a given dimension which is determined by the contact of the upper side of the armature 6 with the damper 10.
In the print head having the arrangement described above, when the coil 5 is energized, the armature 6 is attracted toward the core 4 by means of magnetic attraction force, and when the supply of current to the coil 5 is cut off, the armature 6 is returned by means of the resilient force from the return spring 8 which is compressed when the armature 6 is attracted toward the core 4, thereby activating the wire 7 connected to the armature 6, and thus effecting printing.
When the print head arranged as described above is to be assembled, each armature 6 is inserted into the area between each pair of adjacent retaining portions 11a of the armature guide 11 through the associated return spring 8, and thereafter, the rear case 2 is fitted to the upper end of the front case 1, thereby bringing the support spring 9 and the damper 10 into contact with the upper end of the armature 6 so as to position the armature 6 in place. Accordingly, in the course of the assembling process, the armature 6 inserted into the armature guide 11 may be pushed back by means of the resilient force from the return spring 8 as shown in FIG. 7, resulting in occurrence of problems such as the armature 6 coming off the armature guide 11 or the wire 7 connected to the armature 6 disengaging from the wire guide 1a provided at the distal end of the front case 1.
In order to prevent the occurrence of such problems during the assembling process, if the thickness of the armature guide 11 is increased to prevent the armature 6 from coming off the guide 11 and the length of the wire 7 is increased more than is necessary to prevent the wire 7 from disengaging from the wire guide la, in the former case the thickness of the armature guide 11 is uselessly increased to hinder the reduction in size of the print head, whereas, in the latter case the unnecessary portion of the wire 7 must be cut after the completion of the assembly process, which gives rise to a new disadvantage that the working efficiency is deteriorated.